Virgin cellulosic fiber typically derived from logs of hardwood or softwood, undergoes lengthy processing before it is suitable for use in papermaking. In a typical pulping process, briefly, logs are reduced to wood chips, which are fed into a digester. "Liquor", an aqueous solution obtained from the later described wash step and containing dissolved and residual cooking chemicals, spent cooking chemical residue and cellulosic contaminants, and "white liquor", another by-product of the pulping process known in the art, are fed into the digester, primarily for dilution. Cooking chemicals are also added a required. The cooking chemicals are described hereinafter.
The contents of the digester are brought to a relatively high temperature and pressure, for example about 350.degree. F. at a pressure of about 110 pounds per square inch. The wood chips are "cooked" in the digester under these conditions to reduce the wood chips to pulp. Typically, under these conditions, the wood chips are cooked from about 1 to 5 hours. The cooking can be carried out in batch or continuous digesters.
The cooked wood chips or pulp in the aqueous medium after digestion is referred to as "brown stock". The brown stock consists generally of two phases, the pulp, and the liquor or liquid phase of the digester contents. However, typically after digesting, oversized chips, insufficiently cooked chips, or knots remain. These components are generally removed from the brown stock by knotters which typically consist of coarse screens.
Before further processing of the pulp, it is generally considered necessary to separate the pulp from the liquor. It is also desirable to clean the pulp, removing and to the greatest extent possible, recovering spent or excess cooking chemicals, and removing and recovering pitch contaminants.
After digestion, and following removal of oversized chips and the like, the brown stock is transferred to a washer for a washing step. Typically, the washing process involves a series of washers which separate the pulp from the liquor, and progressively clean the pulp by removal of cooking chemicals, cooking chemical residues, and non-cellulosic contaminants.
Several methods may be used to perform the washing step. In the past, the brown stock was filtered in a false bottom tank or diffuser into which the digester was discharged. The liquor was drained through the false bottom, and the pulp was washed by gravity displacement of the liquor with wash water. Other types of washers such as a pressure washer are also known in the art.
Currently, the rotary vacuum drum or cylinder or vacuum washer is more typically used. As is known to those familiar with the art, the vacuum washer is generally a wire cylinder or drum that rotates in a vat containing the brown stock (i.e. the pulp and liquor mixture). The lower section of the drum is immersed in the brown stock. Vacuum is applied inside the drum as it rotates through the brown stock. The liquor drains through the surface of the wire drum into the interior, leaving a layer of pulp on the outside face of the drum. The layer of pulp is held in place by the vacuum force inside the drum, from where it is conducted away.
The layer of pulp continues to build, forming a mat or sheet, as the submerged portion of the drum rotates through the brown stock in the vat. Liquor continues to drain from the pulp or fiber mat as a result of the differential pressure between the external atmosphere and the vacuum within the cylinder.
Washing action is generally provided by showers located over the pulp sheet. Water is sprayed onto the pulp sheet to displace the liquor from the sheet on the drum as the drum continues to rotate. The vacuum force draws the water into the sheet, where it displaces the liquor. The liquor drains out the other side of the sheet into the inside of the cylinder, where it drains away to a filtrate storage tank for reuse, for example as wash water for a more contaminated sheet which has formed on another of the washers in the series.
Finally, the pulp sheet is removed from the face of the wire by a doctor blade.
The surface of the sheet where the wash water is applied becomes cleaner than the pulp adjacent to the cylinder at the bottom of the sheet, since the wash water becomes more concentrated in liquor as it passes through the sheet. Consequently, where a series of washers is utilized, the pulp sheet obtained from the first vacuum washer is generally repulped to provide a more uniformly clean pulp before traveling over the second vacuum washer. This repulping step is generally repeated between each vacuum washer in the sequence.
In the repulping step, the pulp fibers are agitated at a low consistency (i.e. the pulp is very dilute) in order to facilitate scrubbing. The low consistency also aids in a achieving a lowered concentration of dissolved solids, prior to collection of the pulp on the next washer in the series. Low consistency promotes diffusion of the contaminated liquor from the pulp in the repulping step.
In a sequence of washers, the pulp medium and the wash water are generally arranged to flow countercurrent to each other. Fresh water is typically used to wash the pulp sheet on the last stage washer. The filtrate that was pulled through pulp sheet on each washer is used to wash the pulp on the preceeding washer. This aids in minimizing dilution of the liquor which is separated from the pulp, and from which cooking chemicals or cooking chemical residues are to be recovered, as described hereinafter.
The cooking chemicals used in pulping mills are known in the art. Briefly, the cooking system is generally either kraft or sulfite. Other cooking systems are also known in the art.
The kraft system generally involves the use of sodium hydroxide and sodium sulfide in the digester to aid in decomposition of the wood fibers to produce pulp. The sodium may be added as sodium sulfate, sodium carbonate, or similar sodium compounds. The sulfite system typically involves the use of SO.sub.2 and magnesium, calcium, sodium or ammonia. The kraft mill generates "black liquor", while the counterpart in the sulfite mills is referred to as "red liquor". For the purpose of this description, the term "liquor" refers to both "red" and "black" liquor, and the aqueous phase of the pulp mixture resulting from other pulp processing methods such as those described below.
Some pulping mills form pulp from wood products without the use of cooking chemicals. Several such pulping processes are known, including mechanical processes such as the groundwood process, use of a refiner to create refiner mechanical pulp, or use of heat to create thermomechanical pulp. Most such processes rely on heat and mechanical action to break down the wood fibers. Other processes, such as the NSSC process, rely on both chemical and mechanical action. While these mechanical, thermomechanical, or semichemical processes typically do not involve washing steps, where washing steps are used the methods of this invention can aid in cleaning the pulp and recovering organic contaminants.
Before washing, the brown stock will contain many impurities from the pulping process, including excess cooking chemicals and spent cooking chemicals (where chemicals are used in the pulping), and also a variety of organic contaminants such as resin acids, fatty acid soaps and the like originating in wood. The contaminants occlude to the pulp fibers, and are also present in the aqueous phase of the brown stock. It has been found that in general, the contaminants of black liquor and the corresponding pulp are principally alkali lignin, hydroxy acids and lactones, and sodium. Generally, black liquor is also contaminated with acetic acid, formic acid, sulfur, extractives, and methanol. Red liquor (obtained through the sulfite process) and the corresponding pulp has been found to be contaminated with lignosulfonate, monosaccharides (mannose, xylose, galactose, glucose and arabinose), poly and oligosaccharides, calcium, aldonic acids, sugar-sulfonates, extractives, acetic acid, methanol, and glucuronic acid. These materials are substantially different from those encountered in deinking or dewaxing repulping proceseses, where the contaminants are generally inorganic substances and very different organic compounds.
It is highly desirable to recover or reclaim the cooking chemical residues for reuse, to reduce the amount of chemicals which must be purchased by the mill. Those residues which remain in the pulp after the pulping process are generally not recovered, and contaminate the pulp products. Those residues which are carried by the liquor tend to be recoverable. Therefore, it is advantageous to decrease the amount of chemicals carried by or occluded to the pulp and increase the amount carried by the liquor. In particular, it is desirable to effect a transfer of chemicals from the pulp to the liquor.
Such a transfer can be achieved to a great extent by the washing of the pulp. However, for production level washing to obtain large quantities of pulp at a high quality level or level of purity, vast quantities of wash water are required. The wash water dilutes the liquor and chemicals washed from the pulp. Since recovery of the chemicals involves distillation, or evaporation of the aqueous component, it can be significantly more expensive to recover chemicals from a more dilute solution, off-setting the cost benefits to be achieved by recovery. Thus, a substantial need exists for a method of washing virgin pulp which will sufficiently remove excess and spent cooking chemicals from the pulp and otherwise clean the pulp without causing excessive dilution.
In addition to the problems of recovering inorganic cooking chemical residues, another similar set of second problems encountered in pulping virgin pulp results from the presence of lignin and other organic substances such as resin acids, fatty acid soaps, etc. in wood chips. It is desirable to recover these substances because they are economically or commercially valuable, for example, when recovered as tall oils. In addition, pulp which retains a high level of such materials may require the use of more chemicals in the bleaching step, thus rendering the bleaching step more costly. A need exists for a method of washing virgin pulp which will result in greater recovery of organic substances and lighter colored pulp while minimizing the amount of dilution or wash water required to obtain these results.